1. Field of the Invention
This invention relates to high tilt angle FLC mixtures for TIR switching devices, particularly the high tilt angle compound employed in such mixtures.
2. The Prior Art
Ferroelectric liquid crystals (FLC's) are well known for their ability to switch optical radiation in the the microsecond regime as opposed to conventional nematic liquid crystals, whose response times are generally limited to tens of milliseconds. Schematic diagrams of a typical surface stabilized ferroelectric liquid crystal cell, as first proposed by Clark and Lagerwall are shown in FIGS. 1 and 2; See N. A. Clark and S. T. Lagerwall, "A Microsecond-Speed, Bistable, Threshold-Sensitive Device", in Liquid Crystals of One-and Two-Dimensional Order, W. Helfrich and G. Heppke, Eds, Springer-Verlag, New York, 22 (1980), incorporated herein by reference. The layer planes of the smectic structure are normal to the cell substrates, while the smectic molecules lie parallel to the substrates and make an angle with the layer planes, which is defined as the tilt angle .theta.. Application of a DC electric field causes the molecules to reorient through an angle which is approximately equal to 2.theta.. The value of .theta. is determined by the molecular structure of the smectic liquid crystal molecule and for most materials is typically 19.degree.-23.degree.. If a device containing such a material be placed between crossed polarizers and aligned so that the initial molecular orientation is parallel to one of the polarizers (non-transmissive state), an electric field causes the molecules to rotate through an angle of approximately 45.degree. and the device behaves like a uniaxial wave plate (.DELTA.n=n.sub.e -n.sub.o, transmissive state). Since this type of device configuration is the most commonly employed, numerous liquid crystal mixtures with small tilt angles (19.degree.-23.degree.) are commercially available.
A second type of switching device employs the principle of total internal reflection (TIR) and is shown schematically in FIGS. 3 and 4. This device geometry was first suggested by Soref, for use as a fiber optic switch, and utilized a nematic liquid crystal as the active element. See R. A. Soref, "Low Cross-Talk 2.times.2 Optical Switch", Optics Letters, Vol 6 (1981), incorporated herein by reference. The device exhibits very high contrast ratios (&lt;1000:1), but suffers from the same response time limitations as the previously described nematic devices.
The response time limitation in the above devices can be overcome by replacing the nematic material with a ferroelectric smectic liquid crystal; this has been demonstrated both by the applicant herein and also by Meadows et al. See M. R. Meadows, M. A. Handschy, and N. A. Clark, "Electro-optic Switching using Total Internal Reflection by a Ferroelectric Liquid Crystal", Appl Phys. Lett. 54 (15), 10 Apr. 1989, incorporated herein by reference. The ferroelectric TIR device is shown in FIGS. 5 and 6. An applied DC voltage rotates the molecules through a 90.degree. angle, which changes the perceived refractive index from n.sub.o to n.sub.e and defeats the TIR effect as indicated in FIGS. 3 and 4. For a ferroelectric liquid crystal material to be useful in such a device, it must therefore possess a value of the molecular tilt angle .theta. of approximately 45.degree., since the molecule rotates through an angle approximately equal equal to 2.theta. upon application of a DC field to the cell. This requirement for a large molecular tilt angle eliminates all but a few of the commercially available mixtures from consideration for use in TIR devices.
The prior art has suggested certain FLC compounds; see U.S. Pat. No. 4,892,676 to Sakurai it al and U.S. Pat. No. 4,880,561 to Tabohashi it al (1989) which compounds however, are nitorgen containing.
Accordingly, there is a need and market for improved FLC compounds and mixtures thereof that overcome the above prior art shortcomings.
There have now been discovered certain compounds which serve as highly suitable FLC materials with large molecular tilt angle, which compounds have further been formulated into several mixtures which are highly useful in FLC TIR switching devices.